Posted
by
Soulskill
on Wednesday December 07, 2011 @05:19AM
from the destructive-interference dept.

Hugh Pickens writes "The magnitude-9.0 Tohoku-Oki temblor, the fifth-most powerful quake ever recorded, triggered a tsunami that doubled in intensity over rugged ocean ridges, amplifying its destructive power at landfall, as seen in data from NASA and European radar satellites that captured at least two wave fronts that day, which merged to form a single, double-high wave far out at sea. This wave was capable of traveling long distances without losing power. Ocean ridges and undersea mountain chains pushed the waves together along certain directions from the tsunami's origin. 'It was a one-in-10-million chance that we were able to observe this double wave with satellites,' says study team member Y. Tony Song. 'Researchers have suspected for decades that such 'merging tsunamis' might have been responsible for the 1960 Chilean tsunami that killed about 200 people in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now.' The study suggests scientists may be able to create maps that take into account all undersea topography, even sub-sea ridges and mountains far from shore to help scientists improve tsunami forecasts."

Better understanding and forecasting would be a fantastic thing. It would certainly save lives. However, after watching the footage of the tsunami in question, there's a little part of my brain that wonders whether it would do much more than tweak the odds for people in a few marginal cases.

I suppose where there's a much clearer use for this is in making infrastructure and resilience planning decisions. It will never be practical to say "people shouldn't live in areas that might be hit by tsunamis". As the "Boxing Day" tsunami demonstrated, the areas in question are absolutely vast - and as the Japanese tsunami demonstrated, they can stretch miles inland. I just don't see how countries could afford to give up such huge tracts of habitable land to mitigate against the risk of "once every couple of centuries" events. What might be practical, however, is to think about how to site critical pieces of infrastructure (such as... say... nuclear power plants, as well as hospitals, emergency response centres, transportation hubs) so as to minimise their exposure to these events - and understanding the paths that future tsunamis are likely to follow is going to be key to that.

And protecting your key infrastructure is vital to saving lives in the days and weeks after a catastrophe - particularly in nations less wealthy and less resilient than Japan (which understandably struggled even despite those advantages).

I'm no expert on earthquakes and tsunamis, but, if they have a general idea of locations of faults, or probabilities where earthquakes are highly likely to occur, the mathematics behind determining tsunami combinations might help them better evaluate the risk for any given location. If the old method of thinking or planning was to prepare for a single event, this could help them determine the probability of a merged wave.

What might be practical, however, is to think about how to site critical pieces of infrastructure (such as... say... nuclear power plants...

Honest question - does anyone know why the Fukushima plant was built on the east coast of Japan, facing the bomb-waiting-to-go-off [google.co.uk] that is the massive subduction zone a few miles off shore?

Why wasn't it built on the west coast, so it was sheltered by the island itself? I know hindsight is a wonderful thing, but looking at the map this seems like a schoolboy error to me.

My vague memory from articles at the time is that the Fukushima plant was built before knowledge of tsunamis is as advanced as it is today, and that it had a degree of resilience built in against a "more normal" tsunami, rather than the absolute monster that did appear.

Of course, it's worth remembering that while a lot of people had their lives disrupted, the casualties directly caused by Fukushima were limited. If you're looking for examples of disastrous infrastructure planning decisions with fatal consequences, I'd suggest reading up on the Vajont Dam [wikipedia.org]. I've seen this used in public sector training courses as a cautionary example of what the consequences of rushed, sloppy or biased planning decisions can be. It also, largely through coincidence, involves a tsunami (and one with a much, much greater wave height than that which hit Japan this year) - and a freshwater one to boot.

the Fukushima plant was built before knowledge of tsunamis is as advanced as it is today, and that it had a degree of resilience built in against a "more normal" tsunami, rather than the absolute monster that did appear

Here is a counterpoint [slashdot.org]. Risk management is not a one-time thing: if you are in a risky business (be is security, medical safety, nuclear power, etc.), you need to continually re-assess what risks exist and whether you have properly mitigated them. Failure to take new knowledge, such as improved tsunami science, into account as it becomes available and act on it appropriately is simply negligent.

So where's the evidence that tsunami science had become "available"? And it's worth noting that the link is to an article that says the Tokai plant survived intact only because it was hit by a much smaller tsunami than the Fukushima plant. There's no lesson about risk management there.

I would be willing to be geography has a lot to do with it. Most of the central part of Honshu(the main island) is incredibly mountainous. In order to run power from the west coast to Tokyo and the surrounding prefectures, you would have to run a lot of power lines through those mountains. This would be incredibly difficult as well as incredibly expensive, so my guess is that they try to keep the power generation on the same side of the island as the electricity is used on.

Electric grid of Japan is most likely at fault. It's not unified like in most Western countries, and is instead split between power generating companies. Ability to transfer electricity between different grids is nonexistent.

There is a geographical reason for this: Japan is a very mountainous area, and pulling and maintaining electric cables across mountains is astronomically expensive.

it is split in two parts only; eastern and western Japan, and the split is because the power frequency differs, with 50Hz in the east (including Tokyo and northern Japan) and 60Hz in the west with Osaka, and Kyuushuu and so on. The grids are pretty dense otherwise, but there's only three points capable of converting power between the two grids.

And the reason has nothing to with being mountainous. Simply, as electricity was first introduced, different companies in Tokyo and Osaka bought their power generation stuff from different places, with the Tokyo company buying from Germany (which used 50Hz) and the Osaka one from the US (which used 60Hz). With no regulation in place for the burgeoning industry and no short-term reason for the companies to agree over such things, the country split, electrically, and the split remains today.

There's plenty of power plants along the Japan sea as well. I strongly suspect the placement is dictated more by the availability of a good site and local political willingness to accept a nuclear power plant within the prefecture.

There's plenty of power plants along the Japan sea as well. I strongly suspect the placement is dictated more by the availability of a good site and local political willingness to accept a nuclear power plant within the prefecture.

And the availability of water. Lots and lots of water.

Those towers we think of for nuclear power plants are cooling towers because fission produces a LOT of heat. So much that air cooling isn't enough. So nuclear plants have to be beside a body of water large enough to handle the

Chalk it up to an unfortunate faulty understanding of the geology of the area. Before the quake, it was expected that the largest quake -- and therefore tsunamis that resulted from that quake -- that could come out of that area was right around an 8. So the reactor was designed to survive an 8.5, and the seawall in that region was designed to block an 8-ish tsunami.

However, after watching the footage of the tsunami in question, there's a little part of my brain that wonders whether it would do much more than tweak the odds for people in a few marginal cases.

There are actually quite a few things you can do to prevent tsunami causing that much damage. For example building under-sea groins near the shore that take much of the energy of the wave out of it, reducing its height and speed.

Japan has an extensive network of under-sea sensors to provide early warning of earthquakes and tsunami. Sometimes a warning flashes up on the TV before you feel the tremors (they time time to radiate out from the epicentre), and then a few seconds later they announce if there will

Somewhere there's another awesome animation of a huge tsunami caused by an earthquake off the coast of Chile. The waves travel across the Pacific and swing around Hawaii, which acts like a lens and focuses multiple waves that converge right on Japan. Fun times!

What I find most interesting about tsunami is their ability to move energy so efficiently, with next to no loss. It's a wave traveling in a medium that has very little internal friction. This allows it to travel a tremendous distance without losing a lot of its energy.

The other thing is how it moves the energy when it's out in the ocean. Think of the tsunami when it's out in the middle of the ocean, and is generating a swell of say an inch. It's lifting a two mile column of water an inch up. That's a huge amount of energy.

When that wave approaches land, the distance from sea floor to sea level is much smaller, but the energy is mostly still there. So instead of raising it an inch, you get several meters. They refer to this as "coastal amplification" or something like that, but it's quite the nasty thing to pull when it hits landfall. If you were in a little skiff a few miles off the coast you may not even notice your boat rock, but if you were in that same boat a few hundred feet from the beach you'd be surfing in on a 25 meter high wave. What an amazing difference! It's no wonder Japan got yachts parked on streets a mile in from the coast.

Ocean floor topography plays a role in this too. And then you get effects like waves reflecting off entire continents and other large land masses, and opposite cases where little pockets here and there are sheltered from the effects due random luck involving coastal features. Tsunami are easily just as interesting a study as tornadoes or typhoons in their subtlety and power.